1. Technical Field
The present invention relates generally to composite structural panels fabricated of lightweight, fireproof materials; and, more particularly, to improved lightweight, fire-retardant structural panels of the type used as floor and/or bulkhead panels in, for example, aircraft, ships, submarines and the like wherein the materials employed must meet extremely stringent, and often conflicting, requisites in terms of strength, lightness in weight, ability to resist heat and/or fire, reduction of smoke and toxic fumes generated when subjected to fire, and improved sound deadening characteristics. For example, a field where the present invention finds particularly advantageous use is in the fabrication of floor and/or bulkhead panels for commercial aircraft--panels that are required to be as light as possible and as strong as possible while, at the same time, being capable of meeting and exceeding those standards which are established by regulatory agencies in terms of fire resistance and reduced emission of smoke and/or toxic fumes such, for example, as those fire standards developed, promulgated, and from time to time revised by the FAA and which are more commonly known as regulation "FAR 25"--standards which are constantly being upgraded and made even more rigorous in terms of those structures that will be approved for use in future commercial aircraft, particularly those to be made and introduced into commercial fleets in and/or shortly after 1984/1985.
2. Background Art
The present invention arose specifically in connection with the commercial aircraft industry and is intended to meet and exceed minimum fire resistance standards as established by the FAA--standards which are presently being revised and modified for proposed adoption during or shortly after 1984/1985. At the same time, however, the invention intimately involves other design considerations and constraints which are typically involved in the aircraft industry such, for example, as maximized strength at minimum weight for the particular materials employed, economy, durability, and ability to reduce and minimize noise levels in those regions of the aircraft where noise problems are normally encountered--for example, in the regions where airfoils, power propulsion plants and the like are attached to the aircraft fuselage, in the region of the wheel wells, etc. Thus, the materials employed and the specific constructional details disclosed for panels made in accordance with the present invention provide a synergistic result in terms of reduced weight and cost, reduced propensity to emit smoke and/or toxic fumes, increased strength and durability, increased fire resistance and/or fire retardance characteristics, increased heat and/or sound insulation, and increased versatility in terms of usage in specific environments where the strength required for any given panel may vary from region to region.
As is well known, today's commerical aircraft are subjected to very stringent requirements in terms of ability to withstand fire and to emit minimum quantities of smoke and/or other toxic fumes--standards established by the FAA under code regulation "FAR 25". However, this regulatory standard is continuously undergoing review and revision to improve the safety characteristics of the aircraft; and, at the present time, changes have been proposed for adoption in or shortly after 1984/1985 which will impart even greater and more stringent requirements with regard to design factors having a bearing on fire resistance and safety. The invention is particularly concerned with providing composite panels suitable for use as floor and/or bulkhead panels which will not only meet and exceed the proposed modified standards; but, in addition, which will enhance the strength and durability of the panels, decrease the weight thereof, increase the sound and/or heat insulating characteristics of the panels and, in general, achieve all of the design requirements on an economical basis.
In conventional aircraft available today, it is a common practice to form floor panels of composite sandwich-type construction, generally having a honeycomb core formed of paper treated with a fire resistant material and/or of fire resistant synthetic materials and sandwiched between upper and lower fiberglass face sheets bonded thereto by means of a conventional epoxy-type adhesive. Although the size of such panels is not critical and may be varied to meet desired conditions, the panels are commonly about 4'.times.4' in area and about 1/2" in thickness. However, such panels are commonly employed in different structural environments having different requirements in terms of, for example, load bearing capacity, strength, and/or sound deadening capacity. Thus, where the panels are employed as internal bulkheads, there is often very little load carrying capacity required; and, the degree of sound deadening characteristics required is a function of the location of the panel on the bulkhead--viz., whether the panel is to be employed adjacent a point of attachment of an airfoil, power plant or the like which serves to generate increased noise levels or, alternatively, whether it is to be employed at a region remote from any relatively troublesome sound sources. On the other hand, if the panel is to function as a floor panel, then its load bearing capacity becomes considerably more significant dependent upon whether the floor panel is for a cargo deck or for the passenger deck; and, in the latter instance, whether the panel is located in: (i) a low traffic area such as found beneath the seats in the passenger compartment; (ii) a high traffic area such as the galleys and/or passenger aisles; or (iii), in regions which bridge low and high traffic areas. Again, the particular location of the panel--i.e., whether it is in a region of high noise levels such as adjacent a wheel well or near an equipment area, or whether located in a region of relatively low noise levels--will be determinative of the sound deadening characteristics required.
Prior conventionally available composite panels have either been manufactured to meet the most severe conditions anticipated or have had to be specially fabricated to meet conditions peculiar to the environment within which the panel is to be used. Such panels have not employed a construction which permits any given panel to optimally meet differing design requirements and/or constraints in terms of load bearing capacity and/or sound deadening characteristics which will normally be encountered in the regions to be covered by such panels. Moreover, paper honeycomb cores sandwiched between fiberglass face sheets will not be capable of meeting the revised standards presently being proposed for adoption in FAA fire code regulation "FAR 25" as of 1984/1985.
Numerous approaches to the problems of providing lightweight, fire-retardant composite panels have heretofore been disclosed in the prior art. Typical of the disclosures are those found in, for example, U.S. Pat. Nos. 3,407,110--Axelson, et al.; 3,475,262--Sargent, et al.; 3,600,249--Jackson, et al; 4,061,812--Gilwee, Jr., et al.; 3,713,959--Rottmayer, et al.; 4,344,995--Hammer assigned to the assignee of the present invention; and, 3,778,336--Adams. Thus, both the Axelson, et al. and Sargent, et al. patents relate to heat shields suitable for use with space vehicles and which employ a honeycomb core made of phenolic-glass material (Axelson, et al) into which is added a foamed synthetic material--viz., an elastomeric asbestos fiber in Axelson, et al and various ablative silicone materials in Sargent, et al. Sargent, et al further suggest the use of a face sheet preferably formed of phenolic material. In Jackson, et al, the patentees disclose fiberglass fabrics impregnated with polyimide resins to impart flexibility during expansion of the honeycomb; while Gilwee, Jr., et al disclose honeycomb cores formed of polyamide and a combination of polyimide and glass, with a face sheet of fibrous material impregnated with polyimide resin. In Rottmayer, et al and Hammer, the honeycomb core is bonded to face sheets formed of graphite fibers imbedded in an epoxy matrix (Rottmayer, et al) and graphite reinforced laminates (Hammer). Adams discloses the use of a hexagonal honeycomb core formed of glass fibers impregnated with phenolic resins, and which is pyrolized and dusted with vinylidene fluoride. Other prior patents of miscellaneous interest include U.S. Pat. Nos. 3,811,997--Yuan (impregnated fibrous sheets and honeycomb type structures coated with polyimides); 3,930,085--Pasiuk (a honeycomb or foamed ceramic core with a glass-polyimide laminate); and, 4,299,872--Miguel, et al (intumescent and expanded intumescent materials in combination with a honeycomb core bound to a suitable metal or other laminate).